The most obvious change due to seasonal variation is the change from summer to winter. Some animals migrate to other parts of the world, such as migratory birds. Many animals in colder climates hibernate. In both cases, the absence of these animals leads to a gap in the ecosystem. In regions with highly seasonal rainfall, the low rainfall season can lead to low flows in rivers, and a reduction of certain habitats for some aquatic organisms, such as beaver or aquatic insects.